In this letter, a method to enhance the bandwidth of a double dipole quasi-Yagi antenna using a stepped slotline structure is presented. A stepped slotline with different widths is employed in the coplanar strip line connecting the two dipoles in order to improve impedance matching over a wide frequency band. In addition, two parasitic strip directors are appended to the antenna to enhance the gain in the high-frequency region. A detailed design procedure for the proposed antenna is explained, along with a performance comparison of the input impedance, voltage standing wave ratio (VSWR), and broadside gain. To demonstrate the effectiveness of the proposed design method, a prototype antenna operating in the 1.60-3.60 GHz frequency range with a gain > 6 dBi is designed and fabricated on an FR4 substrate. Experiment results show that the proposed antenna has the desired impedance characteristics with a frequency band of 1.59-3.64 GHz (78.4%) for a VSWR < 2, and a stable gain of 6.4-7.4 dBi in the 1.60-3.60 GHz frequency range. Moreover, a measured front-to-back ratio > 10 dB is obtained. Index Terms-bandwidthenhancement, stepped slotline structure, strip directors, double dipole quasi-Yagi antenna (DDQYA), mobile communications 1536-1225 (c)

A coupled‐sectorial‐loop antenna (CSLA) with circular sectors is presented for super wideband (SWB) applications. Conventional CSLA consists of two parallel sectorial loop antennas connected along an axis of symmetry and triangular sectors are used. The average input impedance of the conventional CSLA is over 100 Ω, and a half of the CSLA above a ground plane is used to match with a 50‐Ω feed line. In the proposed antenna, circular sectors are used instead of triangular sectors to directly match with the 50‐Ω feed line, and the full structure of the CSLA is used. A prototype of the proposed antenna is fabricated on an FR4 substrate, and its performance is compared to the conventional CSLA. The experimental results show that the frequency band for a voltage standing wave ratio < 2.15 is 1.0–19.4 GHz (180.4%), which satisfies the requirement of the SWB antenna. © 2014 Wiley Periodicals, Inc. Microwave Opt Technol Lett 56:1683–1689, 2014

A slot-loaded microstrip patch sensor antenna is proposed to enhance sensitivity in measuring the permittivity of planar materials. A thin rectangular slot was etched along the radiating edge of a rectangular patch antenna fed by a microstrip transmission line. Two resonant frequencies were created at a lower frequency compared to the single resonant frequency of a conventional ordinary patch antenna. The sensitivity of the proposed slot-loaded patch antenna was measured by the shift in the resonant frequency of the input reflection coefficient when the planar dielectric superstrate was placed above the patch, and was compared with that of a conventional patch antenna without the slot. The two antennas were designed and fabricated on a 0.76 mm-thick RF-35 substrate for the first resonant frequency to resonate at 2.5 GHz under unloaded conditions. Five different standard dielectric samples with dielectric constants ranging from 2.17 to 10.2 were tested for sensitivity comparison. The experiment results showed that the measured sensitivity of the proposed patch antenna were 3.54 to 4.53 times higher, compared to a conventional patch antenna, for the five samples.

A broadband series-fed two dipole array antenna is presented for mobile-base station applications covering the frequency bands for 2G, 3G, and long-term evolution systems. Two printed-strip dipole antennas with different lengths backed by a ground reflector are connected through a coplanar stripline, and an integrated balun consisting of a microstrip line and slot line is used to match the input impedance of the antenna to the 50 X feed line. Experimental results show that for a voltage standing wave ratio < 2, the proposed antenna presents a 49% bandwidth in the range of 1.7-2.8 GHz, a stable gain of 5.5-6.3 dBi, and a front-to-back ratio of 12-17 dB. Thus, the proposed antenna is suitable as a broadband base station element antenna, covering all mobile frequency bands.ABSTRACT: In this article, a novel wireless local area network (WLAN) frequency range monopole antenna is designed and manufactured for multi-input-multi-output (MIMO) applications. The proposed antenna consists of three pairs of folded L-shaped strips are placed side by side and printed on a dielectric substrate to generate triple-band operation which is suitable for WLAN applications. In this structure, each pair of L-shaped strips monopole antenna can create additional resonances within the WLAN range, which the desired resonant frequencies are obtained by adjusting the dimension of the folded L-shaped strips. The operating frequencies of the proposed antenna are 2.4/5.2/5.8 GHz which covers WLAN systems frequency range. Various configurations of array of this meander line structure monopole antenna for MIMO application are also studied. Prototypes of the proposed antenna have been constructed and studied experimentally. Good return loss and radiation pattern characteristics are obtained in the frequency band of interest. Simulated and measured results are presented to validate the usefulness of this proposed small antenna structure for MIMO applications.

This study proposes a high-sensitivity microwave sensor based on an interdigital-capacitor-shaped defected ground structure (IDCS-DGS) in a microstrip transmission line for the dielectric characterization of planar materials. The proposed IDCS-DGS was designed by modifying the straight ridge structure of an H-shaped aperture. The proposed sensor was compared with conventional sensors based on a double-ring complementary split ring resonator (CSRR), a single-ring CSRR, and a rotated single-ring CSRR. All the sensors were designed and fabricated on 0.76-mm-thick RF-35 substrate and operated at 1.5 GHz under unloaded conditions. Five different standard dielectric samples with dielectric constants ranging from 2.17 to 10.2 were tested for the sensitivity comparison. The sensitivity of the proposed sensor was measured by the shift in the resonant frequency of the transmission coefficient, and compared with conventional sensors. The experiment results show that the sensitivity of the proposed sensor was two times higher for a low permittivity of 2.17 and it was 1.42 times higher for a high permittivity of 10.2 when compared with the double-ring CSRR-based sensor.